1. Field of the Invention
Embodiments of this invention relate to methods and systems for recovering rare earth elements from materials including rare earth element under mild conditions.
More particular, embodiments of this invention relate to methods and systems for recovering rare earth elements from materials including rare earth element under mild conditions, where the methods and systems include the use of a rare earth element crystallization medium under solvothermal conditions to form rare earth element crystals capable of gravity separation.
2. Description of the Related Art
The Department of Energy has issued recently two reports (Critical Materials Strategy 2010 and an update in 2011) outlining strategies to address critical materials challenges.1,2 The critical materials research and development plan has three major components: diversifying supply, developing substitutes, and improving recycling. Of the fourteen specific elements identified as critical, eight are rare earth elements: lanthanum is a component of fluid catalytic cracking catalysts which are important in petroleum refining, neodymium and dysprosium are used in permanent magnets in wind turbine generators and electric vehicle motors, and cerium, europium, terbium and yttrium are used as phosphors for energy efficient fluorescent lights.
Rare earth metals, particularly lanthanum and cerium are important additives in fluid catalytic cracking catalysts used in petroleum refining to produce gasoline. Catalysts containing rare earths have higher selectivity to gasoline and greater stability in the high-temperature and steam environment associated with catalyst regeneration. The Department of Energy has made an estimate of the consumption of lanthanum oxide in fluid catalytic cracking (FCC). For an FCC unit processing 60,000 barrels/day of feedstock, the catalyst makeup rate to preserve catalyst activity is 4 tons/day.2 Until recent, trends to lower rare earth metal content, typical FCC catalysts contained 3.5% rare earth oxide corresponding to 280 lb per day lanthanum oxide in the makeup catalyst. The total amount of rare earth metal oxides used in catalytic cracking catalysts was 27,000 tons in 2008 of which 66% was lanthanum oxide and 32% was cerium oxide. The other major catalytic use of cerium oxide is in automotive exhaust three-way catalysts for catalytic converters.
Today, spent catalysts are considered hazardous waste and are disposed of in landfills or used to make construction materials. The rare earth oxides are not recovered. The price for lanthanum oxide used has risen from $5 per kilogram in early 2010 to $140 per kilogram in June 2011, and consequently significant incentives exist either to recover rare earths from spent catalysts or to reduce their levels of use.
Most of the chemistry that has been investigated for rare earth recovery involves strong acid digestion of the spent materials followed by some form of solvent extraction of the rare earth component. The use of strong acids both in recycling and in the production of rare earth metals from minerals has significant associated waste disposal issues and adverse environmental impacts. As an example of approaches that have been tried previously, Kulkarni et al.3 developed a method to dissolve rare earth elements from fluidized-bed catalytic cracking (FCC) catalysts that used a high temperature (200° C.), high pressure (200 psig), and acid digestion (HNO3, HF and H3BO3). Other methods include leaching the metals using hydrochloric acid or nitric acid and then extracting La and Ce from the acid systems by extraction into an organic medium containing a complexing agent. Similar approaches have been used for extraction of rare earths from fluorescent lights or CRT phosphors. For example, Rabah used pressure leaching in sulfuric acid/nitric acid mixtures4 and Shimizu et al. used supercritical carbon dioxide containing tri-n-butyl phosphate complexes in HNO3 and H2O for extraction.5 
Thus, there is a significant need in the art for new techniques and technology for the effective and efficient recovery of rare earth metals and/or oxides from spent catalysts and other waste sources without the need of strong acids.